1. Field of the Invention
The current invention relates to a lubrication system including powder-lubricated piston rings for diesel engines, particularly adiabatic diesel engines and coal- water slurry fueled diesel engines.
2. Description of the Prior Art
In order to achieve an adiabatic diesel cycle in an internal combustion engine, heat transfer from the cylinder walls to the cooling system and radiator must be blocked. As such heat transfer normally accounts for 30-35% of the total fuel energy of a diesel cycle, such a configuration results in increased surface temperatures at the cylinder and rings, particularly at the top of the stroke of the piston. As these increased surface temperatures exceed the current limit of 500.degree.-600.degree. F. for the best available synthetic hydrocarbon lubricants, current adiabatic diesel cycles are derated to maintain the temperature levels acceptable for hydrocarbon lubrication. This, of course, results in lower fuel efficiency and lower power per unit weight.
Even in such a derated system, much of the additional energy in the combustion gases resulting from the insulation of the cylinders and resultant low heat rejection in an adiabatic diesel cycle cannot be removed in the primary expansion process in the cylinders. A commonly suggested way of improving efficiency by capturing this additional energy is to use a turbocharger followed by a power turbine coupled to the transmission. The engine then becomes a high-temperature turbomachine similarly requiring a high-temperature lubrication system in order to achieve optimum performance.
Similarly, hydrocarbon-based lubrication systems for coal-water slurry (CWS) or coal-air fueled diesel engines are deficient. While the ability to achieve full combustion and reasonable power levels has been demonstrated for a CWS-fueled diesel cycle, the piston ring/cylinder liner wear rates reported are 20 to 150 times higher than that encountered with conventional fuels. Piston ring wear in CWS-fueled diesel engines is caused by the abrasiveness of the coal ash formed, and in some instances, by the abrasiveness of the coal-water slurry. This high abrasiveness is contributed to by the fact that the ash content of CWS fuels, even with the best existing clean coal technology, ranges from 0.5-1.0%, which is ten to twenty times the ash content in heavy, residual fuel oil. Moreover, liquid lubricants can provide very little protection against wear when the abrasive particle size is such that it is on the same order of magnitude as the oil film thickness. As liquid lubricant requires motion of the lubricated parts to generate a film, wear is particularly acute at the speed reversal points of the piston as the ring/liner contact at these points will be either unlubricated or in the boundary lubricated regime. The resulting high wear rates are a major technological barrier to the full utilization of CWS-fueled diesel technology.
Additionally, CWS-fueled diesel engines have the potential for severe lubrication system malfunctions in the event of the mixing of the water from the slurry with the engine oil with the resultant mixture emulsifying into a gel or in the event of blowby and transfer by the piston rings of the abrasive ash into the crankcase oil.